For operations like this, you're much better off consulting Help Desk
than looking in the MzScheme manual, which is a more spartan document
and reflects an internal split between language and library.
Typing "pretty print" in Help Desk yields the documentation. Note
that PRETTY-PRINT (obtained from (lib "pretty.ss")) appears to be
quite spartan, because it takes only a value and a port. However,
there are numerous parameters that influence the way it works. The
search for "pretty print" yields all of these, so be sure to look into
them if you want to customize its behavior.
Shriram